A general motor driving (or control) circuit generates a motor driving (or control) current by switching of a power device such as a power transistor, for driving a motor with a device consuming a small amount of power. FIG. 1 shows a conventional motor driving circuit using a triangular wave. FIG. 2 shows a driving voltage Vm and a driving current Im supplied to a motor 130 from the motor driving circuit of FIG. 1.
Referring to FIG. 1, the motor driving circuit includes a PWM (pulse width modulation) signal generator 110 and an inverter 120. The PWM signal generator 110 receives and compares an input signal Vi input to a positive input terminal of a comparator through a first resistor and a triangular wave signal Vts input to a negative input terminal of a comparator through a second resistor. The comparator within the PWM signal generator 110 generates a pulse signal.
In a time period where the triangular wave signal Vts is greater than the input signal Vi, the PWM signal generator 110 outputs a negative supply voltage −Vdd. In a time period where the triangular wave signal Vts is less than the input signal Vi, the PWM signal generator 110 outputs a positive supply voltage Vcc. Therefore the PWM signal generator 110 generates the pulse signal.
The inverter 120 includes power transistors Tr1 and Tr2 which are BJTs (bipolar junction transistors) in the example of FIG. 1. The bases of the BJTs Tr1 and Tr2 have the pulse signal from the PWM signal generator 110 applied thereon. The emitters of the BJTs Tr1 and Tr2 are coupled together at a node of the motor 130 having the driving voltage Vm generated thereon. A couple of diodes are connected in series between the collectors of the BJTs Tr1 and Tr2 as illustrated in FIG. 1. The diodes are coupled together at the node of the motor 130 having the driving voltage Vm generated thereon. In addition, a positive terminal of a first power source E is coupled to the collector of the first BJT Tr1, and a negative terminal of a second power source is coupled to the collector of the second BJT Tr2.
Referring to FIGS. 1 and 2, when the PWM signal generator 110 outputs the positive supply voltage Vcc, the first power transistor Tr1 is turned on so that the driving voltage Vm of the motor 130 is the positive power voltage +E. Alternatively, when the PWM signal generator 110 outputs the negative supply voltage −Vdd, the second power transistor Tr2 is turned on so that the driving voltage Vm of the motor 130 is the negative power voltage −E. A driving current Im of the motor 130 increases when the driving voltage Vm has a positive value, and decreases when the driving voltage Vm has a negative value.
In addition, as illustrated in FIG. 2, the driving current Im gradually increases as the pulse width of the driving voltage Vm increases. FIG. 3 illustrates an ideal driving voltage and an ideal driving current with respect to the rotation of a motor. FIG. 3 shows the driving voltage Vm1 and the driving current Im1 with respect to the rotation angle of a motor along the horizontal axis. The driving current Im1 is desired to follow a sine wave form. In other words, when the driving current Im1 in a sine wave form is applied to the motor, the motor rotates according to the Fleming's left hand rule.
In order to generate the driving current Im1 in a sine wave form, the pulse width of the driving voltage Vm1 is desired to vary accordingly as the sine wave form. However, the general motor driving current Im by generating the conventional motor driving circuit may not follow the sine wave form resulting in undesired noise.